1. Field of the Invention
The present invention relates to a scanning optical system, and more particularly relates to a scanning optical system in which a polygonal mirror is enclosed in a housing.
2. Description of the Related Art
In a laser beam printer, laser scanner, or bar-code reader etc., it is necessary to use a scanning optical system. There are two types of scanning optical systems, namely, a transmission type, in which light deflected by a polygonal mirror is transmitted through an f.theta. lens to scan an objective surface, and a reflection type, in which light deflected by a polygonal mirror is reflected by a curved mirror to scan the objective surface. In the reflection type scanning optical system, a curved mirror is employed, and therefore no problems arise due to chromatic aberration. Also, the size of the reflection type scanning optical system is small since the path of light is bent by the curved mirror.
In recent scanning optical systems, the speed of the rotational movement of the polygonal mirror has increased, for example, to increase the printing speed of a laser beam printer. In order to reduce the sound produced by the polygonal mirror during the high speed rotation thereof in the air, and to provide a dust-proof structure, the polygonal mirror is enclosed in a sealed housing.
The sealed housing is at least provided with a transparent plane-parallel plate (glass cover) through which a laser beam is made incident, before being incident upon the polygonal mirror, and from which the laser beam, reflected from the polygonal mirror, is emitted. Part of the laser beam reflected by the plane-parallel plate may be made incident upon the objective surface, thus resulting in a deteriorated print, in the case of a laser beam printer.
To solve this, it has been proposed to arrange the plane-parallel plate of the housing to be not parallel with the reflecting surface of the polygonal mirror within a sub-scanning plane, so that the light reflected by the plane-parallel plate does not reach the objective surface. However, this proposal is applied to a scanning optical system in which the laser beam is made incident upon the polygonal mirror within a main scanning plane, wherein the improvement is addressed to the inclination angle of the plane-parallel plate.